GB2377504A

GB2377504A - Optical filter having coupled birefringent polarisation maintaining waveguides

Info

Publication number: GB2377504A
Application number: GB0117129A
Authority: GB
Inventors: John A R Williams; Wei Zhang; Lai Yicheng
Original assignee: Aston Photonic Technologies Ltd
Current assignee: Aston Photonic Technologies Ltd
Priority date: 2001-07-13
Filing date: 2001-07-13
Publication date: 2003-01-15
Anticipated expiration: 2021-07-13
Also published as: GB2377504B; GB0117129D0

Abstract

An optical filter 10 comprises at least two birefringent polarization maintaining optical waveguides 12,14 for receiving an optical signal of a known linear polarization, the waveguides being optically coupled together in series, the birefringence axes of adjacent waveguides being rotated relative to one another, the optical path length difference between the birefringence axes of the adjacent waveguides being selected to be 2<SP>N-1</SP>( W n L), where N is the number of waveguides, W n is the birefringence of the shortest waveguide, and L is the length of the shortest waveguide, such that, on propagation along the waveguides, the E-field of the optical signal is split into 2<SP>N</SP> optical signal taps, adjacent optical signal taps having substantially equal time delays therebetween, and receiving means 26, in optical communication with the final optical waveguide, operable to combine the signal taps, to thereby form a filtered optical signal. A microwave photonic filter (90, Fig.14) comprises the optical filter above and an optical modulator to superimpose a microwave signal to be filtered onto an optical signal. Receiving means recover the filtered microwave signal.

Description

Optical Filters The present invention relates to an optical filter and to a microwave photomc filter incorporating an optical filter.

Microwave communication links implemented by fibre optics have various advantages over conventional co-axial or waveguide links. These advantages include reduced size, weight and cost, low and constant attenuation over the entire modulation frequency range, imperviousness to electro magnetic interference, extremely wide band width, low dispersion and high information transfer capacity. These advantages mean that they are being considered for a number of applications of commercial importance, such as personal communications networks, millimetre-wave radio LANs, connection to remote antennas at satellite earth stations, broad band video distribution network and signal distribution for phased array antennas.

Recent developments in the performance of practical microwave optical sources have overcome a number of the limitations which prevented these advantages being realised, and has stimulated interest in the use of photonics to implement various functions in microwave systems. This specification addresses further developments in these fields.

According to a first aspect of the present invention there is provided an optical filter comprising: at least two birefringent polarization maintaining optical waveguides for receiving an optical signal of a known linear polarization; the waveguides being optically coupled together in series the birefringence axes of adjacent waveguides being rotated relative to one another, the optical path length difference between the birefringence axes of adjacent waveguides being selected to be 2N-l (An L), where N is the number of waveguides, An is the birefringence of the shortest waveguide, and L is the length of the shortest waveguide,

such that, on propagating along the waveguides, the E-field of the optical signal is split into 21\ optical signal taps, adjacent optical signal taps having substantially equal time delays therebetween ; and receiving means, in optical communication with the final optical waveguide, operable to combine the signal taps, to thereby form a filtered optical signal.

The E-field of one or more optical signal taps may be negated with respect to the E-fields of the other optical signal taps in order to produce a desired optical filter response. The number of optical signal taps which are negated with respect to the other optical signal taps may depend upon the number of optical waveguides.

The angle of relative rotation between the birefringence axes of adjacent waveguides is preferably selected to give the E-fields of the optical signal taps desired magnitudes, to thereby give the optical filter a desired filter response.

The optical filter preferably further comprises polarization selection means, before the first optical waveguide and in optical communication therewith, for receiving an optical signal of unknown polarization and being operable to convert the said optical signal into an optical signal of a known linear polarization. The polarization selection means preferably comprises a polarization controller and a section of polarizing optical waveguide.

The birefringent polarization maintaining optical waveguides preferably each comprise a section of birefringent polarization maintaining optical fibre.

The birefringent polarization maintaining optical fibre is preferably high birefringence optical fibre, and is most preferably two-polarization high birefringence optical fibre.

The polarization controller is desirably an optical fibre based polarization controller.

The birefringent polarization maintaining optical waveguides may be butt-coupled together end-to-end in series. Adjacent sections of optical waveguide may additionally be rotatable relative to one another such that the angle of relative rotation between the birefringence axes of adjacent optical waveguides may be changed by a user, to thereby facilitate adjustment of the magnitudes of the E-fields of the optical signal taps and thus alter the filter response of the optical filter.

Alternatively, the birefringent polarization maintaining optical waveguides may be fusion spliced together end-to-end in series.

The time delay between adjacent optical signal taps is desirably equal to the optical path length difference between the birefringence axes of the shortest birefringent polarization maintaining optical waveguide divided by the speed of light.

Desirably, each birefringent polarization maintaining optical waveguide has the same birefringence value. Each birefringent polarization maintaining optical waveguide preferably comprises a section of the same type of twopolarization high birefringence optical fibre, whereby the optical path length difference between the birefringence axes of adjacent sections of said optical fibre being selected to be An (2N-j L).

The optical filter may further comprise length adjusting means operable to adjust the length of one section of birefringent polarization maintaining optical fibre, to thereby ensure that the optical path length difference between the birefringence axes of adjacent sections of said optical fibre is the selected An (2N ' L). The adjusting means preferably comprises a heater member across which part of the said one section of birefringent polarization maintaining optical fibre is located, such that the said part of the optical fibre is heated, thereby increasing the length of the said section of optical fibre. The heater member is preferably a Peltier device.

The receiving means preferably comprises a square-law detector, such as

a photodetector. The photodetector may form part of an optical spectrum analyser. The receiving means preferably further comprises a section of polarizing optical waveguide optically coupled between the final birefringent polarization maintaining optical waveguide and the square-law detector. The propagation axis of the polarizing optical waveguide is desirably rotated, most preferably by approximately 45 degrees, relative to the birefringence axes of the final birefringent polarization maintaining optical waveguide.

The or each polarizing optical waveguide is desirably a polarizing optical fibre.

According to a second aspect of the present invention there is provided a microwave photonic filter comprising an optical filter according to the first aspect of the invention, an optical source operable to generate an optical signal, and an optical modulator operable to superpose a microwave signal to be filtered onto the optical signal, thereby applying a frequency modulation to the optical signal, the receiving means being further operable to recover the filtered microwave signal.

The optical source is preferably a laser, and may be a tuneable laser.

The optical modulator is desirably an electro-optic modulator.

Specific embodiments of the invention will now be described in detail, by way of example only, with reference to the accompanying drawings, in which: Fig. 1 is a diagrammatic representation of an optical filter according to the first aspect of the invention; Fig. 2 is a diagrammatic representation of the butt coupling apparatus used in the optical filter of Fig. 1; Fig. 3 is a diagrammatic cross-sectional view through the first section of two-polarization high birefringence optical fibre of the optical filter of Fig. 1, showing degeneration of a linearly polarized optical signal into two degenerated

polarized modes; Fig. 4 is a diagrammatic cross-sectional view through the second section of optical fibre of the optical filter of Fig. 1, illustrating the division of each mode from the first section of HiBi optical fibre into the two orthogonal polarized modes of the second section; Fig. 5 is a diagrammatic cross-sectional view through the output polarizing fibre illustrating the coupling of the four degenerated polarization modes from the second section of HiBi fibre into the propagation axis of the polarizing fibre; Fig. 6 is a diagrammatic representation of the sequence of propagation of the four optical signal taps in the polarizing fibre; Fig. 7 is an illustration of the simulated filter response of the optical filter of Fig. 1; Fig. 8 is an illustration of the filter response of the optical filter of Fig. 1 for different angles of relative rotation between the birefringence axes of the input polarizing fibre and the first section of HiBi fibre, the first section of HiBi fibre and the second section of HiBi fibre, and the second section of HiBi fibre and the receiving section of polarizing fibre; Fig. 9 is an illustration of the filter response of an optical filter similar to the optical filter of Fig. 1, having a ratio of the length of the first section of HiBi fibre to the second section of HiBi fibre of 2.002 ; Fig. 10 is a diagrammatic representation of an alternative optical filter according to the first aspect of the invention, being generally similar to the optical filter of Fig. 1, including length adjustment means; Fig. 11 is an illustration of the filter response of the optical filter of Fig. 10, the length adjustment means being in operation; Fig. 12 is a diagrammatic representation of a further alternative optical filter according to the first aspect of the present invention; Fig. 13 is an illustration of the simulated filter response of the optical filter of Fig. 12; and Fig. 14 is a diagrammatic representation of a microwave photonic filter according to the second aspect of the invention.

A first aspect of the present invention provides an optical filter 10, as illustrated in Fig. 1. The optical filter 10 comprises two birefringent polarization maintaining optical waveguides, in the form of a first section of two-polarization high birefringence (HiBi) optical fibre 12 and a second section of HiBi optical fibre 14, for receiving an optical signal of a known linear polarization. The first section of HiBi fibre 12 and the second section of HiBi fibre 14 are optically butt coupled together in series using the butt coupling arrangement 16 shown in Fig. 2, as will be described in more detail below. The HiBi fibre may be fibre type HB1500 (Bow-tie) from Fibrecore Ltd.

The optical filter 10 further comprises polarization selection means 18, for receiving an optical signal of unknown polarization and being operable to convert the optical signal into an optical signal of a known linear polarization which is then transmitted into the first section of HiBi fibre 12. The polarization selection means 18 comprises an optical fibre based polarization controller 20, and a section of polarizing optical fibre 22. The polarization controller 20 is optically coupled to the polarizing fibre 22 by means of a fusion splice 24. The polarization controller 20 is operable to align the polarization of an input optical signal to the propagation axis of the polarizing fibre 22. The construction and operation of suitable polarization controllers is well known to persons skilled in the art and will not be described in detail here.

The optical path length difference between the birefringence axes of adjacent waveguides is selected to be 21'I (AnL). In this example the number of waveguides is 2. Therefore, the optical path length difference (OPD) between the birefringence axes of the first section of HiBi fibre 12 and the second section of HiBi fibre 14 is selected to be 2 (AnL). An is defined as being the birefringence of the shortest waveguide. In this example both the first section of HiBi fibre 12 and the second section of HiBi fibre 14 are the same type of optical fibre and have the same birefringence of approximately 4.5 x dz L is the length of the shortest waveguide. Since the birefringence of the two sections of HiBi fibre 12,14 is the same, the requirement that the OPD between the birefringence axes of the first and second sections of HiBi fibre 12,14 is 2

(AnL) is met when the length of the first section of HiBi fibre 12 is twice the length of the second section of HiBi fibre 14, i. e. the OPD is (2L) An. This will be discussed in more detail below. Alternatively, the length of the second section 12 could be twice the length of the second section 14. The condition could be met with sections of the same length if made of different fibre types, so that An (and thus AnL) is different for each section.

On propagating along the two sections of HiBi fibre 12, 14, the E-field of the input linearly polarized optical signal is split into 2N, in this example 4, optical signal taps. The E-field of one optical signal tap is negated with respect to the E-fields of the other optical signal taps, as will be described in more detail below.

The time delay between each pair of adjacent optical signal taps is substantially the same, the time delay being related to the free spectral range of the filter response, as will be described in more detail below.

The optical filter 10 further comprises receiving means 26 operable to combine the optical signal taps created as a result of the E-field of the linearly polarized optical signal being split during propagation through the first section of HiBi fibre 12 and the second section of HiBi fibre 14, to thereby form a filtered optical signal.

The receiving means 26 in this example takes the form of a second section of polarizing fibre 28, followed by a photodetector 30. The second section of polarizing fibre 28 is optically coupled at one end to the second section of HiBi fibre 14, using the butt coupling arrangement 16. The other end of the second section of polarizing fibre 28 is optically coupled to the photodetector 30. The polarizing fibre 28 and the photodetector 30 together act to combine the optical signal taps, to thereby form a filtered optical signal.

The butt coupling apparatus 16 is shown in more detail in Fig. 2, and comprises a first fibre adaptor 32 and a second fibre adaptor 34. One of the fibre adaptors 32,34 is mounted on a rotatable stage (not shown) such that the

fibre adaptor and the fibre mounted therein may be rotated about the longitudinal axis of the fibre. Each fibre is located within its respective fibre adaptor 32, 34 such that the cleaved end of the fibre is aligned with the output end 32a, 34a of the respective fibre adaptor 32,34. The fibre adaptors 32,34 are arranged such that their respective output ends 32a, 34a, and their respective fibres, come into abutment with each other. The fibres respectively mounted within the fibre adaptors 32, 34 are therefore optically butt coupled to one another.

The angle of relative rotation between the birefringence axes of the first section of HiBi fibre 12 and the second section of HiBi fibre 14 is selected to give the E-fields of the optical signal taps desired magnitudes, to thereby give the optical filter 10 a desired filter response. The use of a butt coupling arrangement 16 enables a user to alter the angle of relative rotation between the birefringence axes of the two sections of HiBi fibre 12, 14. A change in the angle of relative rotation between the birefringence axes results in a change in the magnitudes of the E-fields of the optical signal taps, and thus alters the filter response of the optical filter.

It will be appreciated by the skilled person that instead of two sections of optical fibre being optically butt coupled together, sections of optical fibre may instead be optically coupled to one another in a fixed relationship by means of a fusion splice, which may be produced using a known fusion splicer. The use of fusion splices would prevent the angle of relative rotation between the birefringence axes of, for example the first and sections of HiBi fibre 12,14, being altered, and would thus remove a user's ability to alter the filter response of the optical filter 10. This may be desirable when an optical filter having a fixed filter response is required.

Referring now to Figs. 3 to 6, when a linearly polarized optical signal 36

is launched into a section of HiBi fibre, for example the first section of HiBi fibre 12 of the optical filter 10 shown in Fig. 1, at an angle e 1 with respect to the birefringence axes, labelled x and y in Figs. 3 and 4, the optical signal 36 degenerates into two orthogonal polarized modes 38,40 propagating down the

HiBi fibre 12. Due to the birefringence of the HiBi fibre 12, being the difference between the refractive index of the two birefringence axes of the HiBi fibre 12, the two polarization modes 38,40 experience different optical path lengths on travelling through the HiBi fibre 12. The OPD between the two birefringence axes can be expressed as

where An is the birefringence of the HiBi fibre and I is the length of the HiBi fibre.

As a result of the two polarized modes 38,40 travelling along optical paths of different optical path lengths, on exiting the HiBi fibre 12 there is a time delay between the two polarized modes 38,40, which can be expressed as

where c is the speed of light.

If we denote the magnitude of the E-field of the input optical signal 36 as 1, then the magnitudes of the E-fields of the two degenerated polarization modes 38,40 in the first section of HiBi fibre 12, may be expressed as follows:

On leaving the first section of HiBi fibre 12, the two degenerated polarized modes 38,40 (which are now separated in time) are launched into the second section of HiBi fibre 14 at an angle 82 with respect to the birefringence axes of the second section of HiBi fibre 14, as illustrated in Fig. 4. Each of the two degenerated polarization modes 38,40 from the first section of HiBi fibre 12, degenerate into two orthogonal polarized components as they enter the

second of HiBi fibre 14, to thereby give four components 42,44, 46,48. These four components each propagate in one or other of the degenerated polarization modes of the second fibre, and can thus be considered paired, the pairs having orthogonal polarization, and the members of each pair being separated in time. The magnitudes of the E-fields of the four components 42 to 48 may be expressed as follows :

These four components 42 to 48 constitute the four optical signal taps.

The magnitudes of the E-fields of the four optical signal taps 42 to 48, at the exit end of the second section of HiBi fibre 14, can be altered by changing angles e and e,'In addition, as a result of the transition from the first section of HiBi fibre 12 to the second section of HiBi fibre 14 the E-field of one of the optical signal taps 42 to 48, in this case xp 48, is negated with respect to the Efields of the other optical signal taps 42 to 46.

Following the second section of HiBi fibre 14, the four optical signal taps 42 to 48 are launched into the second section of polarizing fibre 28, as shown in Fig. 5. The polarizing fibre 28 is arranged such that its propagation axis 50 rotated by 450 with respect to the birefringence axes of the second HiBi fibre 14.

The four optical signal taps 42 to 48 then propagate down the second section of polarizing fibre 28, as illustrated in Fig. 6.

As discussed above, because the first and second sections of HiBi fibre 12,14 are the same type of HiBi fibre, the time delays between the four optical signal taps 42 to 48 are determined by the lengths of the first and second sections of HiBi fibre 12,14. The time delay between the polarization modes

propagating down the two birefringence axes of the first section of HiBi fibre 12 may be given by

where L is the length of the first section of HiBi fibre 12. The time delay between polarization modes propagating down the two birefringence axes of the second section of HiBi fibre 14 may be given by

where L2 is the length of the second section of HiBi fibre 14.

In this example, the first section of HiBi fibre 12 is twice as long as the

second section of HiBi fibre 14, i. e. Ll = 2L2 and therefore 1dl = 21d2'The four optical signal taps 42 to 48 will therefore be evenly spaced in time, as may be seen in Fig. 6.

The filter response of the optical filter 10 can be analysed as follows:

where the optical signal tap weightings w, are given by:

JE, denotes the optical power registered by the photodetector 30 at the output

of the optical filter 10, n is the average refractive index, and L, is the optical path length which the ph optical tap signal travels.

The filter response of the optical filter 10, for various tap weightings w can be simulated based on the above equation. More intuitively, the filter response may be determined based on the values of 81 and e,, from which the corresponding tap weightings w, are derived.

Fig. 7 shows the filter response of the optical filter 10 when the length of the first section of HiBi fibre 12 is 4. 5m and the length of the second section of HiBi fibre 14 is 2. 25m, with 81 set at 150 and 82 set at 5 go. Each section of polarizing fibre 22,28 has a length of 2m, The normallsed tap weightings are as follows:

W, = 0. 26, w2 =-0. 43, w, = 0. 11 and w = 0. 07 1 4 The simulated filter response shown in Fig. 7 shows that the optical filter 10 has flattened pass bands 52 having a band width of 0. 35nm and a free spectral range of 1. 61nm. The side mode suppression is better than 35dB.

Fig. 8 shows two filter responses of an alternative optical filter according to the first aspect of the invention, which is of the same construction as the optical filter 10 of Fig. 1, but has a first section of HiBi fibre 12 of a length 4. 5301m and a second section of HiBi fibre 14 of length 2.2646m.

As would be expected, when the propagation axis of the first section of polarizing fibre 22 is aligned with one of the birefringence axes of the first section of HiBi fibre 12, the birefringence axes of the first section of HiBi fibre 12 are aligned with the birefringence axes of the second section of HiBi fibre 14, and the propagation axis of the second section of polarizing fibre 28 is rotated by 450 with respect to the birefringence axes of the second section of HiBi fibre 14, the optical filter response 54 is flat.

When the propagation axis of the first section of polarizing fibre 22 is

aligned with one of the birefringence axes of the first section of HiBi fibre 12, the birefringence axes of the second section of HiBi fibre 14 are rotated by 45 with respect to the birefringence axes of the first section of HiBi fibre 12, and the propagation axis of the second section of polarizing fibre 28 is rotated by 450 with respect of the birefringence axes of the second section of HiBi fibre 14, two degenerated orthogonal polarization modes are created within the second section of HiBi fibre 14. The two polarization modes form two optical signal taps, and result in the optical filter 10 having a notch filter response 56. The notch filter response 56 has a free spectral range of approximately 1.6nm.

The creation of four components, i. e. four optical signal taps, begins in the region of an angle of rotation between the propagation axis of the first section of polarizing fibre 22 and the birefringence axes of the first section of HiBi fibre 12 of approximately 14 , an angle of relative rotation between the birefringence axes of the first and second sections of HiBi fibre 12,14 of approximately 58 , and an angle of relative rotation between the birefringence axes of the second section of HiBi fibre 14 and the propagation axis of the second section of polarizing fibre 28 of 450.

Fig. 9 shows the simulated optical filter response (dotted line) and the experimentally recorded optical filter response (solid line) of an optical filter of the same construction as the optical filter 10 of Fig. 1, in which the ratio of the length of the first section of HiBi fibre 12 to the length of the second section of HiBi fibre 14 is 2.002. The propagation axis of the first section of polarizing fibre 22 is rotated by 140 with respect to the birefringence axes of the first section of HiBi fibre 12, the birefringence axes of the second section of HiBi fibre 14 are rotated by 58 relative to the birefringence axes of the first section of HiBi fibre 12, and the propagation axis of the second section of polarizing fibre 28 is rotated by 450 relative to the birefringence axes of the second section of HiBi fibre 14. As a result of the ratio of the length of the first section of HiBi fibre 12 to the length of the second section of HiBi fibre 14 not being exactly two, as required, the filter response 58 exhibits a side mode suppression of only

approximately 11 dB. In addition, the pass bands of the filter response 58 include ripples 60 having an amplitude of 0. 35dB.

Fig. 10 shows a further alternative optical filter 70 which is of similar construction to the optical filter 10 shown in Fig. 1, with the following modification. The same reference numerals are retained for corresponding features.

In this optical filter 70 length adjusting means, in the form of a Peltier heating device 72, are provided. A 25cm length 74 of the first section of HiBi fibre 12 is looped around the Peltier device 72. The temperature of the Peltier device 72 is controlled to better than 0. 1'C using a temperature controller (not shown). By heating the 25cm length 74 of the first section of HiBi fibre 12 the overall length of the first section of HiBi 12 can be increased. In this example, the 25cm length 74 of the first section of HiBi 12 is heated to 4 C above room temperature in order to achieve the desired ratio of the length of the first section of HiBi fibre 12 to the second section of HiBi fibre 14 of two.

The angles of relative rotation between the propagation axis of the first polarizing fibre 22 and the birefringence axes of the first section of HiBi fibre 12, the birefringence axes of the first and second sections of HiBi fibre 12,14, and the propagation axis of the second section of HiBi fibre 28 and the birefringence axes of the second section of HiBi fibre 14 are as described above in relation to Fig. 9.

The filter response 76 of the optical filter 70 is shown in Fig. 11. The side mode suppression has been increased to approximately 30dB, and the ripples on the pass bands have been reduced in amplitude to less than 0.2dB. The flattened pass bands have a band width of approximately 0. 35nm and the filter response has a free spectral range of approximately 1. 6nm.

Fig. 12 shows a further alternative optical filter 80 according to the first aspect of the invention. The optical filter 80 is of similar construction to the optical filter 10 of Fig. 1, with the following modifications. The same reference numerals are retained for corresponding features.

In this example, three sections of HiBi fibre 82,84, 86 are provided, and

are optically coupled together in series as described above. The first section of HiBi fibre 82 has a length of 4m, the second section of HiBi fibre 84 has a length of 2m and the third section of HiBi fibre 86 has a length of 1m.

In this example the angle of relative rotation between the propagation axis of the first section of polarizing fibre 22 and the birefringence axes of the

first section of HiBi fibre 82 is 82 , the angle of relative rotation between the birefringence axes of the first and second sections of HiBi fibre 82, 84 is 820, the angle of relative rotation between the birefringence axes of the second and third sections of HiBi fibre 84, 86 is 450, and the angle of relative rotation between the propagation axis of the second section of polarizing fibre 28 and the birefringence axes of the third section of HiBi fibre 86 is 450.

The optical filter 80, where N = 3 creates 8 optical signal taps. The normalised tap weightings, in order of increasing delay, are 0. 049,-0. 049,-0. 35, - 0. 35, 2. 491, -2. 491, 0. 35 and 0. 35.

The simulated filter response 88 of the optical filter 80 is illustrated in Fig. 13. The filter response 88 exhibits a pass band width of 0.9nm, a free spectral range of 3. 1nm and side mode suppression of greater than 40dB.

A second aspect of the present invention provides a microwave photonic filter 90, as shown in Fig. 14. The microwave filter 90 comprises an optical filter 10 according to the first aspect of the invention, as illustrated in Fig. 1. The microwave filter 90 also comprises an optical source operable to generate an optical signal, which in this example takes the form of a tuneable laser 92, such as the Photonetics Tunics 1550. The microwave filter 90 also comprises an optical modulator, which in this example is an electro-optic modulator 94, operable up to 20GHz. The electro-optic modulator 94 is operable to superpose a microwave signal to be filtered, indicated by arrow 96, onto the optical signal from the tuneable laser 92. The superposition of the microwave signal onto the optical signal, through the electro-optic modulator 94, applies a frequency modulation to the optical signal.

The optical filter 10 operates as described above, with the modification that the receiving means 26 is further operable to recover a filtered microwave signal.

In this example, the electro-optic modulator 94 and the photodetector 30 form part of a lightwave component analyser, not shown, such as the Hewlett Packard HP8703A. The lightwave component analyser may be used to analyse the recovered filtered microwave signal.

It will be known to persons skilled in the art that when an optical signal is frequency modulated, for example at a modulation frequency of IOGHZ, optical side bands are produced on the optical signal at a frequency spacing from the original optical signal of + and-the modulation frequency, i. e. ± 10GHz. A more complex modulation signal will give rise to side bands on the optical carrier spaced above and below the carrier frequency. The optical spectrum of the frequency modulated optical signal is therefore related to the modulating microwave signal frequency content.

The frequency modulated optical signal is then launched into the optical filter 10. The optical filter 10 operates as described above, and may be given a desired filter response as illustrated in Figs. 7 to 9, 11 and 13. The optical filter 10 therefore filters the frequency modulated optical signal, causing some wavelengths in the frequency modulated optical signal to be filtered out.

Because the different wavelengths of the frequency modulated optical signal correspond to different frequencies of the microwave signal, the removal of certain wavelengths from the frequency modulated optical signal by the optical filter 10 results in the removal of the corresponding microwave signal frequencies. The microwave signal is therefore also filtered, as a result of the frequency modulated optical signal propagating through the optical filter 10.

When the filtered optical signal is received by the receiving means 26, the filtered microwave signal is demodulated from the optical carrier signal, thereby recovering the filtered microwave signal.

The pass band width and the free spectral range of the microwave photonic filter 90 are determined by the pass band width and free spectral range of the optical filter 10. For example, referring to the optical filter response 76 illustrated in Fig. 11, a microwave photonic filter 90 including an optical filter 10 having such an optical filter response 76 would have a microwave filter response having a free spectral range of 200GHz with a flattened pass band width of 47. 3 5GHz.

If all of the fibre lengths within the optical filter 10 having an optical filter response 76 as illustrated in Fig. 11 are multiplied by 10, the corresponding frequencies of the microwave photonic filter 90 would be divided by 10, resulting in a microwave filter having a pass band width of 4.7GHz and a free spectral range of 20GHz.

The optical filter 10 within the microwave photonic filter 90 effectively simultaneously filters the optical signal and the modulating microwave signal 96. The microwave photonic filter 90 therefore exploits the use of an optical carrier signal on which a microwave signal 96 to be filtered is carried, to achieve filtering of the microwave signal as a result of filtering of the optical carrier signal by the optical filter 10. The filtering of the microwave signal is therefore achieved using optical processing.

The optical filter 10 is designed such that the time delay between adjacent optical signal taps 42 to 48 results in the optical filter 10 having a frequency/wavelength response within the microwave scale. That is to say, the free spectral range and the band width of the pass bands of the optical filter 10 are, in frequency terms, within the microwave frequency range.

The above examples provide an optical filter based on cascaded sections of birefringent polarization maintaining optical waveguides, such as HiBi optical fibre. Optical signal taps are derived from the degenerated polarization modes travelling down the HiBi fibres. The time delays between signal taps are determined by the length of the HiBi fibres used. Adjustments to the tap weightings, i. e. the magnitude of the E-field of each optical signal tap, can be

achieved by altering the angle of relative rotation between the birefringence axes of adjacent waveguides. Negative weightings of optical signal taps are generated optically. It is not essential to provide at least one tap with negative weighting, but the ability to do so, greatly expands the range of filter responses which can be created.

The optical signal taps are grouped, with time delays, into degenerated, non-interfering, orthogonal polarization modes propagating along the same optical waveguides, therefore the optical filter is not limited by the optical coherence of the optical source, and thus the optical filter overcomes the stringent requirements of conventional coherent optical systems regarding the stability of the delay paths against environmental perturbations.

The microwave photonic filter has all of the advantages of the optical filter.

Various modifications may be made without departing from the scope of the present invention. For example, a different type of birefringent polarization maintaining optical waveguide may be used in place of the HiBi fibre described.

The optical waveguides may be optically coupled together using a different coupling arrangement to that described, or they may be fusion spliced together.

It will be appreciated that an optical filter having more than the described two or three sections of birefringent polarization maintaining optical waveguides may be constructed, having a consequently larger number of optical signal taps.

It will also be appreciated that the optical filters described may be constructed to have different optical filter responses to those illustrated.

Regarding the microwave photonic filter, it will be appreciated that the optical filter response, and thus the microwave filter response, of the optical filter may be different to that described. An optical source other than a tuneable laser may be used, and provision may be made for adjusting the length

of the various components, in order to tune or trim the arrangement to the wavelength of the source, at least to some degree, to arrange for the optical wavelength to fall in a passband in the filter profile. It is envisaged that in many instances, it may be more convenient or appropriate to tune the arrangement to the source, rather than tuning the source to the arrangement.

The electro-optic modulator may be replaced by a different type of optical modulator. The microwave signal to be filtered may have a different frequency and frequency band width to that described.

Whilst endeavouring in the foregoing specification to draw attention to those features of the invention believed to be of particular importance it should be understood that the Applicant claims protection in respect of any patentable feature or combination of features hereinbefore referred to and/or shown in the drawings whether or not particular emphasis has been placed thereon.

Claims

1. An optical filter comprising : at least two birefringent polarization maintaining optical waveguides for receiving an optical signal of a known linear polarization ; the waveguides being optically coupled together in series the birefringence axes of adjacent waveguides being rotated relative to one another; the optical path length difference between the birefringence axes of the adjacent waveguides being selected to be 2N-1 (An L), where N is the number of waveguides, An is the birefringence of the shortest waveguide, and L is the length of the shortest waveguide, such that, on propagation along the waveguides, the E-field of the optical signal is split into 2N optical signal taps, adjacent optical signal taps having substantially equal time delays therebetween; and receiving means, in optical communication with the final optical waveguide, operable to combine the signal taps, to thereby form a filtered optical signal.

2. An optical filter as claimed in claim 1, in which the E-field of one or more optical signal taps is negated with respect to the E-fields of the other optical signal taps is order to produce a desired optical filter response.

3. An optical filter as ; claimed in claim 2, in which the number of optical signal taps which are negated with respect to the other optical signal taps depends upon the number of optical waveguides.

4. An optical filter as claimed in any preceding claim, in which the angle of relative rotation between the birefringence axes of adjacent waveguides is selected to give the E-fields of the optical signal taps desired magnitudes, to thereby give the optical filter a desired filter response

5. An optical filter as claimed in any preceding claim, in which the optical filter further comprises polarization selection means, before the first optical waveguide and

in optical communication therewith, for receiving an optical signal of unknown polarization and being operable to convert the said optical signal into an optical signal of a known linear polarization.

6. An optical filter as claimed in claim 5, in which the polarization selection means comprises a polarization controller and a section of polarizing optical waveguide.

7. An optical filter as claimed in any preceding claim, in which the birefringent polarization maintaining optical waveguides each comprise a section of birefringent polarization maintaining optical fibre.

8. An optical filter as claimed in claim 7, in which the birefringent polarization maintaining optical fibre is high birefringence optical fibre, such as two-polarization high birefringence optical fibre.

9. An optical filter as claimed in any of claims 6 to 8, in which the polarization controller is an optical fibre based polarization controller.

10. An optical filter as claimed in any preceding claim, in which the birefringent polarization maintaining optical waveguides are butt-coupled together end-to-end in series.

11. An optical filter as claimed in claim 10, in which adjacent sections of optical waveguide are rotatable relative to one another such that the angle of relative rotation between the birefringence axes of adjacent optical waveguides may be changed by a user, to thereby facilitate adjustment of the magnitudes of the E-fields of the optical signal taps and thus alter the filter response of the optical filter.

12. An optical filter as claimed in any of claims 1 to 9, in which the birefringent polarization maintaining optical waveguides are fusion spliced together end-to-end in series.

13. An optical filter as claimed in any preceding claim, in which the time delay between adjacent optical signal taps is equal to the optical path length difference

between the birefringence axes of the shortest birefringent polarization maintaining optical waveguide divided by the speed of light.

14. An optical filter as claimed in any preceding claim, in which each birefringent polarization maintaining optical waveguide has the same birefringence value.

15. An optical filter as claimed in claim 14, in which each birefringent polarization maintaining optical waveguide comprises a section of the same type of twopolarization high birefringence optical fibre, the optical path length difference between

the birefringence axes of adjacent sections of said optical fibre being selected to be An (2N-1 L).

16. An optical filter as claimed in claim 15, in which the optical filter further comprises length adjusting means operable to adjust the length of one section of birefringent polarization maintaining optical fibre, to thereby ensure that the optical path length difference between the birefringence axes of adjacent sections of said optical fibre is the selected An (2 N-1 L).

17. An optical filter as claimed in claim 16, in which the adjusting means comprises a heater member across which part of the said one section of birefringent polarization maintaining optical fibre is located, such that the said part of the optical fibre is heated, thereby increasing the length of the said section of optical fibre.

18. An optical filter eis claimed in claim 17, in which the heater member is a Peltier device.

19. An optical filter as claimed in any preceding claim, in which the receiving means is a square-law detector, such as a photodetector.

20. An optical filter as claimed in claim 19, in which the photodetector forms part of an optical spectrum analyser.

21. An optical filter as claimed in claims 19 or 20, in which the receiving means further comprises a section of polarizing optical waveguide optically coupled between

the final birefringent polarization maintaining optical waveguide and the square-law detector.

22. An optical filter as claimed in claimed 21, in which the propagation axis of the polarizing optical waveguide is rotated, by approximately 45 degrees, relative to the birefringence axes of the final birefringent polarization maintaining optical waveguide.

23. An optical filter as claimed in claim 21 or 22, in which the or each polarizing optical waveguide is a polarizing optical fibre.

24. A microwave photonic filter comprising: an optical filter as claimed in any preceding claim ; an optical source operable to generate an optical signal, and an optical modulator operable to superpose a microwave signal to filtered onto the optical signal, thereby applying a frequency modulation to the optical signal, the receiving means being further operable to recover the filtered microwave signal.

25. A microwave photonic filter as claimed in claim 24, in which the optical source

is a laser.

26. A microwave photonic filter as claimed in claim 25, in which the laser is a tuneable laser.

27. A microwave photonic filter as claimed in any of claims 24 to 26, in which the optical modulator is an electro-optic modulator.

28. An optical filter substantially as described above with reference to Figures 1 to 13 of the accompanying drawings.

29. A microwave photonic filter substantially as described above with reference to the accompanying drawings.